Differential magnetic proximity sensor

ABSTRACT

A distance measuring device (100) is provided, comprising a first sensing module (110), a second sensing module (120), a reference device (130), and an evaluating module (140). The first sensing module and the second sensing module are arranged on a horizontal base line (150). Each one of the first and second sensing module is configured to detect the strength of a magnetic field (50) and each one of the first and second sensing module has a first sensing direction (y) and a second sensing direction (x).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/086,406, filed on Sep. 19, 2018, entitled “Differential MagneticProximity Sensor,” which is a § 371 U.S. National Stage Application ofInternational Patent Application No. PCT/EP2017/056428, filed Mar. 17,2017, which claims priority to EP16161508 filed Mar. 22, 2016, bothentitled “Differential Magnetic Proximity Sensor,” the contents of eachof which are hereby incorporated by reference in their entireties forall purposes.

TECHNICAL FIELD

The present technology relates to a distance measuring device which maybe a distance measurement sensor, for example, and an air spring withsuch a distance measuring device.

TECHNICAL BACKGROUND

Height or distance measurement has a wide variety of possibleapplications. For instance, it is a parameter that frequently needs tobe monitored to optimize the performance of various types of machineryand vehicles, such as automobiles, trucks, trains, agriculturalvehicles, mining vehicles, construction vehicles, and the like. Forinstance, monitoring height and various distances can lead to reducedfuel consumption, improved comfort, reduced overall cost, extendedproduct service life, and safety. In any case, the need to monitor suchdistance parameters may generally increase with sophistication of thedevice and the complexity of its features.

Virtually every aspect of complex machinery may need to be tightlymonitored and controlled to attain maximum advantages. For instance,constant adaptations may be required to optimize the performances andefficiency of almost every moving part of the machinery. This typicallyneeds to be done while the operational conditions in the environment ofthe equipment are subject to change and can change significantly oververy short time frames. Changing environmental conditions are virtuallyalways encountered by vehicle. In addition to this, vehicles frequentlyoperate under changing conditions which can make monitoring a difficultchallenge. For instance, monitoring suspension height by distancemeasurements between air spring components can yield useful information.However, the environment where the height measurement is being made canpresent a wide variety of challenges. For example, in measuring theheight of a vehicle frame above the surface of a road, challenges aretypically presented by road noise, dirt, dust, and vibrations which arenormally present in the environment surrounding the vehicle where themeasurement is being taken.

SUMMARY

There may be a need to reduce the effect of unwanted magnetic strayfields on the distance measurement using magnetic fields.

According to an aspect of the technology, a distance measuring device isprovided. The distance measuring device comprises a first sensingmodule, a second sensing module, a reference device, and an evaluatingmodule. The first sensing module and the second sensing module arearranged on a horizontal base line. Each one of the first and secondsensing module is configured to detect the strength of a magnetic fieldand each one of the first and second sensing module has a first sensingdirection and a second sensing direction. The reference device ismovable with respect to the first sensing module and the second sensingmodule along a movement trajectory. The reference device comprises amagnetic field element configured to emit a magnetic field detectable bythe first and second sensing module, wherein the first sensing module isconfigured to detect the magnetic field and to determine a first openingangle between the second sensing direction of the first sensing moduleand the position of the reference device as a result of the detectedmagnetic field strength at the first and second sensing directions,wherein the second sensing module is configured to detect the magneticfield and to determine a second opening angle between the second sensingdirection of the second sensing module and the position of the referencedevice, wherein the evaluating module is configured to determine thedistance between the base line and the reference device based on thefirst opening angle and the second opening angle.

In other words, the first and the second sensing modules are laterallyoffset with respect to the reference device and an opening angle can bedetected between each one of the sensing modules and the referencedevice. These opening angles can be used to determine the distancebetween the sensing modules and the reference device. The first andsecond sensing modules are laterally offset along the horizontal baseline. The vertical distance between each one of the sensing modules andthe reference device is preferably the same. The vertical distance isthe distance parallel to the measuring direction.

It should be understood that a movement of the reference device withrespect to the sensing modules may be a relative movement between thesensing modules and the reference device and may mean that the referencedevice is moved while the sensing modules are standing still or that thesensing modules are moved while the reference device is standing stillor that both, the sensing modules and the reference device are moved.

Due to the fact that the distance between the reference device and thesensing modules is determined based on the opening angles between thereference device and each one of the sensing modules, the effect ofunwanted magnetic stray fields or interfering magnetic fields like forexample the earth magnetic field may be reduced.

The first sensing module may have a first zero angle measurement axispoint and the second sensing module may have a second zero anglemeasurement axis point. The horizontal base line preferably connects thefirst and second zero angle measurement axis points.

Preferably, the first sensing direction and the second sensing directionare substantially orthogonal to each other and preferably intersect atan angle between 20° and 100°. In a preferred embodiment, the first andsecond sensing directions intersect at 90°.

Preferably, the movement trajectory is perpendicular or substantiallyperpendicular to the horizontal base line.

According to an embodiment of the technology, the movement trajectory islinear and preferably an extension of the movement trajectory intersectswith the base line, wherein the base line preferably is a virtual linearline interconnecting the first and second sensing modules.

In other words, the reference device moves towards and/or away from thesensing modules. This movement is referred to as a relative movementbetween said components.

According to a further embodiment of the technology, the movementtrajectory intersects with the base line at a center point which isequidistant from the first sensing module and the second sensing module.

Hence, the first and second sensing modules are laterally offset withrespect to the reference device (or with respect to a center point ofthe reference device) along the horizontal baseline such that thesensing modules have the same distance from the line of the movementtrajectory or from the center of the reference device. In other words,the sensing modules of the distance measuring device are arrangedsymmetrical, i.e., axisymmetric, with respect to the movementtrajectory.

According to a further embodiment of the technology, the first sensingmodule is arranged such that its second sensing direction is inclinedwith respect to the movement trajectory, and/or the second sensingmodule is arranged such that its second detection direction is inclinedwith respect to the movement trajectory.

Thus, the effect of magnetic stray fields, in particular uniformmagnetic stray fields, can be cancelled by this arrangement of thesensing modules.

According to a further embodiment of the technology, the angle ofinclination of the first sensing module is the same as the angle ofinclination of the second sensing module. Hence, the first and secondsensing modules point in the same direction, i.e., they are aligned toeach other.

According to a further embodiment of the technology, the second sensingdirection of the first sensing module and/or of the second sensingmodule is inclined with respect to the movement trajectory at an anglebetween 1° and 89°, preferably between 5° and 70°, more preferablybetween 10° and 45°.

According to a further embodiment of the technology, the second sensingdirection of the first sensing module is parallel to the second sensingdirection of the second sensing module.

According to a further embodiment of the technology, the evaluatingmodule is configured to determine the absolute value of the differencebetween the first opening angle and the second opening angle and todetermine the distance between the reference device and the base linebased on said absolute value.

According to a further embodiment of the technology, the referencedevice comprises a first permanent magnet.

According to a further embodiment of the technology, the first permanentmagnet has a magnetic pole axis which coincides with the movementtrajectory. In other words, the magnetic pole axis is directed towardsthe sensing modules.

According to a further embodiment of the technology, the referencedevice additionally comprises a second permanent magnet, wherein thefirst permanent magnet and the second permanent magnet are locatedequidistantly spaced apart from a vertical center axis of the referencedevice.

The vertical center axis of the reference device may coincide with themovement trajectory.

According to a further embodiment of the technology, a virtual lineinterconnecting a magnetic pole axis of the first permanent magnet and amagnetic pole axis of the second permanent magnet is parallel to thebase line.

This virtual line may be referred to as reference device line. Hence,the first and second permanent magnet may be laterally offset withrespect to the vertical center axis of the reference device along thereference device line.

According to a further embodiment of the technology, a pole axis of thefirst permanent magnet and/or a pole axis of the second permanent magnetis inclined with respect to the movement trajectory at a firstinclination angle and a second inclination angle, respectively.

In other words, the pole axis of the first permanent magnet and thesecond permanent magnet is not perpendicular with respect to thereference device line and the horizontal base line interconnecting thefirst and second sensing modules.

According to a further embodiment of the technology, an absolute valueof the first inclination angle of the first permanent magnet is the sameas an absolute value of the second inclination angle of the secondpermanent magnet.

According to a further embodiment of the technology, the firstinclination angle is between 1° and 25°, preferably between 5° and 20°,more preferably between 10° and 15°, more preferably 15°.

According to a further aspect of the technology, an air spring isprovided. The air spring comprises a first mounting plate, a secondmounting plate, and a distance measuring device. The first mountingplate is adapted to be mounted to a chassis of a vehicle and the secondmounting plate is adapted to be mounted to a wheel suspension. Thedistance measuring device is a device as described above andhereinafter. The first and second sensing modules are mounted to thefirst mounting plate, and the reference device is mounted to the secondmounting plate, wherein preferably the air spring further comprises aflexible member, wherein the first mounting plate, the second mountingplate, and the flexible member define a pressurizable chamber, andwherein the first and second sensing modules and the reference deviceare situated within the pressurizable chamber.

These and other aspects of the present technology will become apparentfrom and elucidated with reference to the exemplary embodimentsdescribed hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a first and a second sensing module.

FIG. 2 schematically shows a first and a second sensing module arrangedwithin a magnetic stray field.

FIG. 3 schematically shows a first and a second sensing module arrangedwithin a magnetic stray field.

FIG. 4 schematically shows a first and a second sensing module.

FIG. 5 schematically shows a distance measuring device according to anexemplary embodiment of the technology.

FIG. 6 schematically shows a movement trajectory of a reference devicewith respect to a base line in connection with a distance measuringdevice according to an exemplary embodiment of the technology.

FIG. 7 schematically shows a distance measuring device according to anexemplary embodiment of the technology.

FIG. 8 schematically shows a first and a second sensing module of adistance measuring device according to an exemplary embodiment of thetechnology.

FIG. 9 schematically shows a distance measuring device according to anexemplary embodiment of the technology.

FIG. 10 schematically shows a reference device of a distance measuringdevice according to an exemplary embodiment of the technology.

FIG. 11 schematically shows a reference device of a distance measuringdevice according to an exemplary embodiment of the technology.

FIG. 12 schematically shows an evaluating module of a distance measuringdevice according to an exemplary embodiment of the technology.

FIG. 13 schematically shows a distance measuring device according to anexemplary embodiment of the technology.

FIG. 14 schematically shows a distance measuring device according to anexemplary embodiment of the technology.

FIG. 15 schematically shows a distance measuring device according to anexemplary embodiment of the technology.

FIG. 16 schematically shows a distance measuring device according to anexemplary embodiment of the technology.

FIG. 17 schematically shows a distance measuring device according to anexemplary embodiment of the technology.

FIG. 18 schematically shows an air spring according to an exemplaryembodiment of the technology.

FIG. 19 schematically shows a wheel suspension with an air springaccording to an exemplary embodiment of the technology.

DETAILED DESCRIPTION

For purposes hereof it should be understood that in referring todistances between two points the points are a first point (base point,or point from where the measurement will start, may be the first andsecond sensing modules as referred to above and hereinafter) and asecond point (target point, may be the reference device as referred toabove and hereinafter) to which the distance is measured. When aimingfor a non-contact distance measurement solution, and when placing thedistance sensing system at the base point, then the used measurementsystem has to be able to physically “detect”, “feel”, or “sense” thetarget point, in some way. There may be a multitude of fundamentaldifferent ways to accomplish this purpose. Some of these solutions canbe optically based (such as visible light, and invisible light), soundbased (for instance, audible and non-audible sounds) or physical basedmeasurements. The measurement solution which is best suited for aspecific application may depend on many factors, including:environmental conditions (interfering lights, interfering sound,changing ambient pressure, temperature, dust, and humidity), spaceavailability for the measurement system, the targeted measurement range(millimeters, meters, kilometers), required measurement resolution andabsolute accuracy, cost limitations, and the like.

In one embodiment, the herein described distance measurement solution isspecifically directed to pneumatic powered, air-spring applications. Itis applicable to the air springs which are employed in a wide variety ofapplications including, but not limited to machinery and vehicles, suchas automobiles, trucks, trains, agricultural vehicles, mining vehicles,construction vehicles, and the like. Even though the followingdescription refers to the specific application of air springs, it shouldbe noted that this field of application is provided for purpose ofexample only and that the distance measuring device may also be used inconnection with other applications.

The air-spring design to which this description is applicable includes aflexible member (an elastic rubber belly) that is mounted in anair-tight manner onto top and bottom plates to define an air tight(pressurizable) chamber. By pumping pressured air into the pressurizablechamber the air-spring will expand and by releasing the air from thepressurizable chamber the air-spring will begin to collapse. Usuallymechanically controlled or electrically controlled pneumatic valves areused to change the amount of air within the pressurizable chamber of theair spring.

The total maximum distance that needs to be measured is equivalent tothe working stroke range of the air-spring. The total working stroke ofan air-spring is the difference in distance between when the air-springis fully expanded (the maximal working length of the air-spring) andwhen the air-spring is fully contracted (the shortest possible workinglength of the air-spring). In other words, this working stroke is thechanges in length of the air-spring when fully pumped-up (maximumpractical air-volume within the air-spring belly) and when almost all ofthe air inside the air-spring has been pumped-out (lowest practicalair-volume within the air-spring belly). The term “air” as used in thiscontext includes any fluids, in particular gas or mixtures of gasseswhich is inert to the air spring and includes air, nitrogen, helium,other Noble gases, nitrogen enhanced air and helium enhanced air, forexample. In particular, the term “air” when referring to an air springmay be understood as a synonym for any compressible gas.

For purposes hereof the targeted distance measurement is typicallywithin the range of a few millimeters to around 500 millimeters or evenmore. The targeted measurement resolution and measurement repeatabilityis typically within the range of about 1 mm to 5 mm. It may be veryinconvenient and may increase costs in scenarios where air-tightpassages need to be tooled into the top or bottom plate of the airspring to accommodate electric cables for electric power supply or otherpurposes. Additionally, air-tight connectors of any type are expensiveand will typically have an adverse effect on the reliability of theair-springs utilizing such technology.

The sensing solution of this description will operate on magneticprinciples as they are not substantially affected by light, sound,air-pressure, dust, and/or humidity. The sensor system of thistechnology may be described as consisting three main parts: (1) thesensing module (or Magnetic Field Sensor Array), the sensor electronics,and the target-point. The sensing module and the sensing electronics areconnected with each other by a number of insulated electrical wires (forexample four wires can be utilized). The sensing module can be placed atthe one end of the air-spring and can be referred to as the base-point.The sensor electronics can be powered by a low DC (direct current)voltage. The target-point or reference device may be a small and highstrength permanent magnet. The physical dimension and the absolutesurface-magnetic-field-strengths of the permanent magnet are subject toa number of application dictated parameters, including the measurementdistance to be covered, available space, and environmental factors,including ferro-magnetic objects that may be situated near to themeasurement path. For purposes hereof the “measurement path” is avertical straight line between the target-point and the base point. Ingeneral, larger more powerful permanent magnets are needed with largermeasurement distances with stronger surface-magnetic-field-strengthsbeing required. In any case, the area around the measurement should befree of moving ferro-magnetic objects as they can interfere negativelywith the distance measurement to be taken. However, within limits,static (not moving) ferro-magnetic objects can be tolerated withappropriate correction factors.

This description is focusing on non-contact proximity measurement usinga magnetic principle based differential mode measurement solution.

The reference device referred to in this description may in particularbe one or more permanent magnets. Thus, the reference device does notneed any energy supply or any other external connections. However, dueto aging of the permanent magnet, the distance measuring device may haveto meet the following requirements: ability to deal with changes in themagnetic field strength (aging, low cost product); limited accuracy inthe actual orientation of the North-South magnetic pole axis (whenhaving to deal with low cost magnets, for example); ability ofcompensating the effects of interfering magnetic stray fields (like theEarth Magnetic Field); maximum freedom in the final placement of theactual sensing device and the reference device and not having to rely onstrict geometrical (rectangular shaped) design requirements; simplemeasurement data processing (fast signal processing using low current,slow operating, and low cost processors).

In many distance measuring applications, the sensor system can be seenas a single axis (X) measurement system or as a two axes (X and Y, alsoreferred to as “2D”) measurement system. There are a few applicationswhere the distance measurement system has to operate in a three axesenvironment (X, Y, and Z, also referred to as 3D). A three axismeasurement system is more commonly referred to as a “position” sensorsystem.

The approach described here can be used for all three application types:Singe Axis, Dual Axes (2D), and Tripe Axes (3D, or position) SensorSolutions. However, the design explanations in this document focusalmost entirely on a Dual Axes Distance Measurement Sensor which,however, should not be understood as a limitation.

FIG. 1 shows two 2-Axis Magnetic Field Sensor Arrays (MFSA 1 and MFSA 2)which are referred to as a first sensing module 110 and a second sensingmodule 120 and which are placed side-by-side with some distance 112 toeach other. The two 2D-MFSA devices, i.e., the first and the secondsensing modules 110, 120, measure the magnetic vectors in the sameplane. The output of each Magnetic Field Sensor Array is a vector thatdescribes the magnetic field intensity measured and the angulardirection from where the magnetic field measured is coming from. Theangular direction may be understood as the position of the magneticfield source relative to each one of the first and second sensing module110, 120 within the coordinate system spanned by the axes x and yindicated in FIG. 1. The axes x and y are also referred to as firstsensing direction (y) and second sensing direction (x).

FIG. 2 schematically indicates the effect of a uniform magnetic strayfield (like the Earth Magnetic Field, indicated by dashed lines 10) thatis hitting both MFSA devices 110, 120. In this case, both MFSA deviceswill report the same vector angle of the magnetic field passing by: TheAngle 1 a from the MFSA 1 will be identical to the Angle 2 a, measuredby the MFSA 2 device.

Angle 1a=Angle 2a

When subtracting the vector angle measurements from each other than theeffect of the uniform magnetic stray field has been cancelled:

Angle 1a−Angle 2a=0

To achieve the cancellation effect and to avoid or eliminate anyunwanted effects of the magnetic stray field on the measured distance itdoes not matter in which way the “zero” angle axis of the MFSA devicesare pointing, as long as they are pointing in the same direction. Inother words, the default orientation of the first and second sensingmodules does not affect the cancellation in a negative manner as long asthe sensing modules are oriented in the same direction.

As shown in FIG. 3, both sensing modules 110, 120 have been turnedaround in clock wise direction by a few degrees. However, the “zero”angle measurement axes (that may be one of the first sensing direction yand the second sensing direction x or any defined direction withreference to the first or second sensing direction) from both MFSA maypoint in the same direction, i.e., the first sensing direction of thefirst sensing module 110 is parallel to the first sensing direction ofthe second sensing module 120.

Due to the angular rotation of the sensing modules 110, 120 shown inFIG. 3 and compared to the sensing modules shown in FIG. 2, the absolutevalue of the angular measurement of the uniform magnetic stray field(Angle 1 b and Angle 2 b in FIG. 3) will now be smaller (in comparisonto the earlier example, Angle 1 a and Angle 2 a in FIG. 2). However,angles 1 b and 2 b are identical again and when subtracting them fromeach other the remaining value will be zero. Meaning that the effect ofthe uniform magnetic stray field has been cancelled again.

Angle 1b−Angle 2b=0

This is the basic design concept of the differential magnetic fieldsensor device that will be used to measure the distance to the referencedevice. Further, the first and second sensing modules may be rotated inclockwise or counter-clockwise direction as to be adapted to thegeometrical circumstances within an air spring. Therefore, the distancemeasuring device as described herein may be used in a flexible mannerand may be adapted to various air spring designs.

FIG. 4 shows the first and second sensing modules 110, 120 withreference to the base line 150. The base line 150 is a virtual linewhich interconnects the magnetic zero-reference points 111, 121 of thesensing modules. The magnetic zero-reference point is the zero-point ofthe y-axis and of the x-axis, i.e., the cross-point of these axes. Inother words, the sensing modules 110, 120 are positioned at the sameheight, i.e., their zero-reference point is at the height of the baseline. The two magnetic field sensor arrays are oriented in the samedirection. In other words, the y-axes of the first and second sensingmodules intersect the base line 150 at the same angle. The same appliesto the x-axes, since the x-axes typically intersect the y-axes at 90°.The zero-angle measurement axis of both sensing modules points into thesame direction. The two magnetic measurement zero-point (magneticzero-reference points 111, 121) of the sensing modules are connected byan imaginary line which is the base line 150.

FIG. 5 shows a distance measuring device 100 with a first sensing module110 and a second sensing module 120 which are positioned at the sameheight at the base line 150. A reference device 130 is arranged oppositeto the sensing modules 110, 120 and is configured to move along amovement trajectory 160 towards and away from the sensing modules, asindicated by the arrows. The reference device is arranged between thetwo sensing modules, i.e., it has a lateral offset with respect to bothsensing modules. In other words, the angular direction of the referencedevice with respect to any one of the sensing modules is neither 0° nor90°, but some angle in between. Depending on the position of thereference device, i.e., the distance from the base line 150, the anglesmeasured by the sensing modules vary. Based on the angular valuesmeasured by the sensing modules, the position of the reference devicecan be determined, and therefore, the distance between the referencedevice and the base line can also be determined.

The reference device will travel towards and away from the base line ina nearly perpendicular manner. The reference device may be a permanentmagnet with its flux-lines facing towards the center of the two MFSAs.In other words, the central axis of the reference device isperpendicular to the base line and intersects the baseline at a pointwhich is equidistant from the first and the second sensing module. Themagnetic directions of the flux lines (North-to-South or South-to-North)does not affect the measurement performance of this sensor design. Thesame applies to the absolute signal magnetic field strengths of thereference device, which is also of little importance. However, thelarger the magnetic field strength may be, the larger the measurementdistance can be.

It should be noted that the actual movement of the reference device mustnot follow the linear movement trajectory shown in FIG. 5. The referencedevice may move on any kind of trajectory. However, the sensing moduleswill measure the distance of the reference device from the base line,which distance is the spacing along the linear movement trajectory.

The base line 150 interconnects the two sensing modules 110, 120. Theaxis or path of reference device 130 moving towards and away from thebase line 150 is here called the movement trajectory 160 or trajectoryline. One end of the trajectory line is connected to the base line. Whenusing 2D (two axes) measuring MFSA's as the first and second sensingmodules, then the movement path of the reference device (the movementtrajectory) and the base line may have to be in the same two-dimensionalplane.

However, when the reference device is not moving in a straight linetowards and away from the base line, and when the plane in which thereference device is moving and/or the movement axis of the referencedevice is not in the same plane as used by the two MFSA's and it isrequired to determine not just the distance between the reference deviceand the base line but the exact positioning of the reference device withrespect to the first and second sensing modules in the three-dimensionalspace, then the MFSA have to be able to measure in three axes (like 3Dmeasuring MEMS, micro electro mechanical systems). This scenario isshown in FIG. 6. The plane in which the base line 150 is located and theplane in which the moving trajectory 160 is located are not the same butintersect at an angle unequal to 0°.

FIG. 7 shows an exemplary embodiment of a distance measuring device. Thezero reference points of the two sensing modules 110, 120 are placed onthe same base line 150. There is a distance 112 between the two zeroreference points, while there is a measured (or calculated) distance 170between the reference device 130 and the base line 150. The distance 170is the measured distance or height.

The reference device is moving along the movement trajectory. Thistrajectory can be straight or can be curved. The end-point of themovement trajectory can end-up anywhere on the base line. However, whencertain conditions are met, then the measurement signal processing canbe very simple and does not require any complex trigonometriccalculations. Simplified operational conditions of the reference devicemovement trajectory may be: the movement trajectory is a straight line(not curved in any direction) and ends perpendicular (in a 90 degrees'angle) onto the base line and the intersecting point of the movementtrajectory and the base line is exactly halve way between the twosensing modules.

FIG. 8 schematically shows possible relative positions of the referencedevice with respect to the sensing modules.

The area around the two sensing modules 110, 120 can be divided in sixrectangular shaped sections 181, 182, 183, 184, 185, 186 that lie in thesame plane as the measurement plane of the sensing modules. The baseline interconnecting the two sensing modules is building the centralhorizontal line between the sections 181, 182, 183 (above the base line)and the sections 184, 185, 186 (below the base line). As long as thereference device is moving within one and the same area (like, forexample, in Area 181 which is marked in FIG. 8), the here describeddistance measuring device can determine the distance of the referencedevice to the base line.

The largest measurement range (longest achievable distance from the baseline) may be achieved if the reference device is located in the sections182 and 185.

FIG. 9 shows a distance measuring device 100 with a reference device 130having two permanent magnets 132 and 134. The absolute magnetic fieldstrength and the physical size of the reference device may be some ofthe design parameters that influence the achievable measurement range.However, the measurement range may also be influenced by the sensitivityof the sensing modules and the distance between the sensing modules aswell as by the magnetic interferences and/or the operational conditions.

The reference device may comprise a single permanent magnet or twopermanent magnets. In the latter case, each one of the permanent magnetsmay be smaller compared to the single-magnet-solution. In particular,the measurement range may be extended by placing two permanent magnetsside by side, as shown in FIG. 9.

The two permanent magnets 132 and 134 are arranged side by side along areference device line 136 which preferably is parallel to the base line150. The reference device line 136 interconnects the center of thepermanent magnets 132 and 134. A central vertical axis of the referencedevice is equidistant to each one of the permanent magnets 132 and 134.This central vertical axis may preferably correspond to the movementtrajectory and may intersect the base line 150 at a point which isequidistant to the sensing modules.

FIGS. 10 and 11 show an additional approach which may increase themeasurement range. The permanent magnets 132, 134 may be tilted withreference to the movement trajectory such that there is an angle 137A,138A at which the central axes 133, 135 of the permanent magnets 132,134 intersect with the movement trajectory 160 or angles 137B, 138B atwhich the central axes 133, 135 intersect with the reference device line136.

Tilting the permanent magnets may help that the magnetic flux linesrunning towards the sensing modules extend even further. To achieve asymmetrical and straight line measurement curve, the two permanentmagnets may have identical specifications (size and strength) and theangles of inclination may be of the same absolute value, but in theopposite direction, i.e., the magnets are inclined towards each other,as shown in FIGS. 10 and 11.

The inwards tilting may be done symmetrically. The optimal tilting leveldepends on the targeted measurement range, the physical magneticstrength of the permanent magnets, the height—diameter ratio of theactual magnet, and the distance between the two magnets. For mostapplications the suitable inwards tilting range (for each magnet) isfrom 0 degree to less than 20 degrees. Going beyond the upper tiltinglimit may rapidly reduce the distance sensor performances and thereforemay be undesirable. The permanent magnets may be tilted by around 15degrees, for example.

FIG. 12 shows an evaluating module 140. The evaluating module 140comprises two sensor array interfaces 144, 145 for interconnecting thefirst and the second sensing module. Further, the evaluating module 140comprises a controller 143, for example a processor or any other kind ofautomated calculation unit, and a power supply unit 141. The powersupply unit 141 may provide the evaluating module with electrical energyreceived from an energy source (not shown) via power interface 146. Thecontrol may send or receive data via an input/output (I/O) interface 142which may comprise a serial digital I/O, an analogue I/O, and/or aninterface to be connected to a data bus, for example a digital bus likeCAN or CAN-Open.

The sensing modules are configured to provide the absolute magneticfield strength measured and the angle where this signal is coming fromor is going to, i.e., the position of the magnetic field source. Thisinformation is transmitted to the evaluating unit 140 via the interfaces144, 145.

The controller 143 will request the 2D Vector Signals of the measuredmagnetic field, whereby only the angular information may be ofimportance. Depending on the sensor system design the simplest form ofsignal processing may be applicable which is building the average of thetwo angle values provided by the two sensing modules and then apply asignal linearization procedure and a conversion from angular measurementin an actual distance value. Both can be done without any computation byusing a lookup table. However, the calculation may also be done by thecontroller 143. The average angular value (Angle 1−Angle 2)/2 will beconverted into a distance value by extracting this value from the lookuptable or by calculating the value. The distance value depends on theangular values measured by the sensing modules and the distance betweenthe two sensing modules.

FIG. 13 exemplarily shows the approach of distance computation. A simplesignal computation can be applied when the reference device 130 ismoving on a straight line, i.e., when the movement trajectory 160 islinear and oriented in a nearly perpendicular fashion towards and awayfrom the base line 150. The movement axis of the reference device willbe nearly halve-way between the two sensing modules 110, 120. Thus, thedistances 112A and 112B are the same and half of the distance 112.

When the reference device 130 is moving along the movement trajectory160 then one of the easiest ways of computing the measurement data is:subtracting angle 1 from angle 2 (meaning the angles measured by thefirst and second sensing module) and using a lookup table to convert theresulting angle into a distance information.

As can be seen in FIG. 13, the first and second sensing directions ofboth sensing modules are inclined with respect to the movementtrajectory and the base line at an angle unequal to 0° and 90°,respectively.

FIG. 14 shows the magnetic flux lines of the reference device 130. Inone exemplary embodiment, the magnetic flux lines 50 generated by thereference device 130 are detected by the first and second sensingmodules 110, 120 in opposite direction. In comparison, the magnetic fluxlines 10 generated by uniform magnetic stray field source like the earthmagnetic field are detected by the first and second sensing modules 110,120 pointing in the same direction. That is why the differential modewill cancel the effects caused by uniform magnetic stray fields, andwill not cancel the signal caused by the reference device.

FIG. 15 shows a distance measuring device 100, wherein the angularvalues between the sensing modules and the reference device areschematically indicated.

In this schematic illustration, two different vector angles measured bythe first sensing module 110) (160°) and measured by the second sensingmodule 120 (200°) are shown.

Angie 1a−Angle 2a=160°−200°=−40°

When subtracting the measured angles from each other (building thedifference), then the remaining result is −40° in this example. Thisdifferential angular value is used as an indicator for the distancebetween the reference device and the base line.

FIG. 16 schematically shows the effect of uniform magnetic stray fieldson the angular measurement of the sensing modules 110, 120. Uniformmagnetic stray fields 10 will affect the angle measurements of themagnetic field emitted by the reference device. Depending on thestrength of the interfering stray field 10, the effect may be larger orsmaller.

Starting from the values shown in FIG. 15, in the schematic illustrationof FIG. 16 the reported angle measurements by the first sensing module110)(150° and by the second sensing module 120 (190° have changed underthe influence of the interfering magnetic stray field.

Angle 1a−Angle 2a=150°−190°=−40°

However, when applying the same mathematical process (building thedifference) the remaining angle value is the same: −40°. In other words,the uniform magnetic stray field influences the measurement of bothsensing modules in the same manner and will be eliminated when applyingthe differential mode.

The cancellation of uniform magnetic stray fields may work best when thereference device is in close range to the sensing modules. In any case,the here described differential mode signal processing is greatlyreducing the unwanted effects from uniform magnetic stray fields.

FIG. 17 shows a distance measuring device 100, wherein the referencedevice 130 is arranged in an alternative manner in comparison to theembodiment shown in FIGS. 13 to 16. The here described sensor systemdesign will also function when the flux lines 50 run horizontally(parallel to the base line) through the reference device.

Some features of the distance measuring device described herein may besummed up as follows:

The magnetic field source (reference device) is defined by the physicaldimensions of the reference device, in particular its height and width,and its absolute magnetic field strength. There may be one or morepermanent magnets used as the magnetic field source. The sensing modulesmay be defined and described by the sensitivity of the individualmagnetic field sensing devices and the absolute distance between thefirst and second sensing modules. The movement trajectory of thereference device may be perpendicular (or angled) with respect to thebase line and may be arranged symmetrically (in the middle) between thefirst and second sensing module. The movement trajectory may be astraight line or a curved line and the reference device may stay within(or not within) the boundaries of one of the six possible sections 181to 186. The interfering magnetic stray fields may be described by themagnetic field strength and by the uniformity/non-uniformity of thestray field.

In one preferred embodiment, the magnetic field source (referencedevice) comprises two permanent magnets, placed side-by-side, laterallyoffset and tilted with respect to the central axis of the referencedevice. The permanent magnets are of identical design and specificationsand the physical dimensions may be defined by a height versus diameterratio between <1 and >0.2. The sensing modules may be arranged such thatthe distance between the first sensing modules and the second sensingmodule is a function of the targeted measurement range and thespecification of the reference device.

The distance measuring device may be used as a height sensor for airsprings (in trucks, passenger cars, and other industrial applications),for determining valve positions, as manufacturing processing equipment,for determining a truck-trailer position (e.g., during the couplingprocess or as a steering assistance to avoid jack-knifing when driving atruck-trailer backwards), and/or for automatic object height measurementin sorting machinery.

It may be of little to no importance what the absolute magnetic fieldstrength of the permanent magnet is. The measuring device describedherein may be immune to any aging effect of the permanent magnet andenables measuring the distance in a non-contact fashion. The sensingmodules can be 2D or 3D axes Hall-Effect solid state sensors with ananalogue signal output or a serial digital bus interface (like I2C orSPI, for example). The measuring device enables much flexibilityrelating to the way the sensing modules and the reference device areplaced relative to each other. This allows easy adaptation to differentapplications where space may be restricted. The space between thesensing modules and the reference device can be filled with any type ofsubstance that has no Ferro-magnetic properties (including Aluminum,sand, wood, plaster, dust, water, oil, etc.). The magnetic flux-lineorientation of the magnetic field source (in relation to the base line)can be any of the four possible directions: North-to-South,South-to-West, South-to-North, and West-to-East. However, bestmeasurement performance and compensation for uniform magnetic strayfields may be achieved when the flux lines point towards the base line(solution North-to-South or South-to-North). The angular reference point(0° Angle) of the sensing modules can point in any direction as long asboth do the same. There is no need for any calibration procedure.

FIG. 18 shows an air spring 200 with a first mounting element 210, asecond mounting element 220, and a flexible member 230, for example abellow. The first mounting element in form of a top plate, the secondmounting element in form of a bottom plate, and the bellow contain orinclude a volume which is the pressurizable chamber 240. The first andsecond sensing modules 110, 120 are arranged at the first mountingelement 210 and the reference device 130 is arranged at the secondmounting element 220 opposite to the first mounting element. There is arelative movement of the mounting elements 210, 220 along the movementtrajectory 160.

In an operating mode of the air spring, the top plate and the bottomplate may move towards each other along the direction arrow 160 bymovements of the bottom plate and/or by movements of the top plate.

FIG. 19 illustrates a wheel suspension 320 and a vehicle's chassis 310,which are mechanically linked to each other and have an air spring 100for dampening vibrations of the wheel 325 due to uneven road conditions,wherein one of the mounting elements of the air spring is mounted to thewheel suspension 320 and the other one of the mounting elements of theair spring is mounted to the vehicle's chassis 310.

The wheel suspension 320 may move along the arrow 322 when the wheelrolls over an uneven street and, as a result of the vibrations of thewheel 325 and of the wheel suspension 320, the mounting elements of theair spring are moving frequently towards and away from each other asindicated by arrow 160. The air spring and in particular thepressurizable chamber within the air spring is adapted to dampen thevibrations of both the wheel suspension and the vehicle's chassis as tonot transfer or transmit these vibrations from one of these parts to theother one, respectively.

It should be understood that the features described in individualexemplary embodiments may also be combined with each other in order toobtain a more fail safe air spring height sensor or air spring as wellas to enable error detection and correction of the measured heightsignal. While certain representative embodiments and details have beenshown for the purpose of illustrating the subject technology, it will beapparent to those skilled in this art that various changes andmodifications can be made therein without departing from the scope ofthe subject technology.

LIST OF REFERENCE SIGNS

10 magnetic stray field

50 magnetic field generated by the reference device

100 distance measuring device

110 first sensing module

111 magnetic zero-reference point

112 distance between first and second sensing module

120 second sensing module

121 magnetic zero reference point

130 reference device

132 permanent magnet

133 central axis

134 permanent magnet

135 central axis

136 reference device line

137 angle of inclination

138 angle of inclination

140 evaluating module

141 power supply unit

142 I/O interface

143 controller

144 sensor array interface

145 sensor array interface

146 power interface

150 base line

155 angle between base line and movement trajectory

160 movement trajectory

170 measured distance

180 measurement plane

181 first section

182 second section

183 third section

184 fourth section

185 fifth section

186 sixth section

y first sensing direction

x second sensing direction

1 a first opening angle

2 a second opening angle

200 air spring

210 first mounting plate

220 second mounting plate

230 flexible member

240 pressurizable chamber

310 chassis

320 wheel suspension

322 suspension movement

325 wheel

What is claimed is:
 1. A distance measuring device (100) comprising: afirst sensing module (110) having a first zero angle measurement axispoint, a second sensing module (120) having a second zero anglemeasurement axis point, a reference device (130), an evaluating module(140), a horizontal base line (150) connecting the first zero anglemeasurement axis point and the second zero angle measurement axis point,wherein each one of the first and second sensing module is configured todetect the strength of a magnetic field (50) in a first sensingdirection (y) and a second sensing direction (x), wherein the referencedevice is movable with respect to the first sensing module and thesecond sensing module along a movement trajectory (160), wherein thereference device emits a magnetic field (50) detectable by the first andsecond sensing module, wherein the first sensing module is configured todetect the magnetic field and to determine a first opening angle (1 a)between the second sensing direction (x) and the position of thereference device as a result of the detected magnetic field strength,wherein the second sensing module is configured to detect the magneticfield and to determine a second opening angle (2 a) between the secondsensing direction (x) and the position of the reference device, whereinthe evaluating module is configured to determine the distance betweenthe base line and the reference device based on the first opening angleand the second opening angle.
 2. The distance measuring device of claim1, wherein the movement trajectory is linear, wherein an extension ofthe movement trajectory intersects with the base line, and wherein thebase line is a virtual linear line interconnecting the first and secondsensing modules.
 3. The distance measuring device of claim 2, whereinthe movement trajectory intersects with the base line at a center point,wherein the center point is equidistant from the first sensing moduleand the second sensing module.
 4. The distance measuring device of claim2 or 3, wherein the first sensing module (110) is arranged such that itssecond sensing direction (x) is inclined with respect to the movementtrajectory (160), and/or wherein the second sensing module (120) isarranged such that its second detection direction (x) is inclined withrespect to the movement trajectory.
 5. The distance measuring device ofany one of claims 1 to 4, wherein the second sensing direction (x) ofthe first sensing module (MFSA1) and/or of the second sensing module(MFSA2) is inclined with respect to the movement trajectory at an anglebetween 1° and 89°.
 6. The distance measuring device of any one ofclaims 1 to 5, wherein the second sensing direction (x) of the firstsensing module (MFSA1) is parallel to the second sensing direction (x)of the second sensing module (MFSA2).
 7. The distance measuring deviceof any one of claims 1 to 6, wherein the evaluating module is configuredto determine an absolute value of the difference between the firstopening angle and the second opening angle and to determine the distancebetween the reference device and the base line based on said absolutevalue.
 8. The distance measuring device of any one of claims 1 to 7,wherein the reference device comprises a first permanent magnet.
 9. Thedistance measuring device of claim 8, wherein the first permanent magnethas a magnetic pole axis which coincides with the movement trajectory.10. The distance measuring device of claim 8, wherein the referencedevice further comprises a second permanent magnet, wherein the firstpermanent magnet and the second permanent magnet are locatedequidistantly spaced apart from the movement trajectory.
 11. Thedistance measuring device of claim 10, wherein a virtual lineinterconnecting a magnetic pole axis of the first permanent magnet and amagnetic pole axis of the second permanent magnet is parallel to thebase line.
 12. The distance measuring device of claim 10 or 11, whereina pole axis of the first permanent magnet and/or a pole axis of thesecond permanent magnet is inclined with respect to the movementtrajectory at a first inclination angle and a second inclination angle,respectively.
 13. The distance measuring device of claim 12, wherein anabsolute value of the first inclination angle of the first permanentmagnet is the same as an absolute value of the second inclination angleof the second permanent magnet.
 14. The distance measuring device ofclaim 12 or 13, wherein the first inclination angle is between 1° and25°.
 15. An air spring comprising: a first mounting plate being adaptedto be mounted to a chassis of a vehicle, a second mounting plate beingadapted to be mounted to a wheel suspension, and a distance measuringdevice according to any one of claims 1 to 14, wherein the first andsecond sensing modules (MFSA1, MFSA2) are mounted to the first mountingplate, and wherein the reference device is mounted to the secondmounting plate, wherein the air spring further comprises a flexiblemember, wherein the first mounting plate, the second mounting plate, andthe flexible member define a pressurizable chamber, and wherein thefirst and second sensing modules and the reference device are situatedwithin the pressurizable chamber.